1. Field of the invention:
This invention relates to a semiconductor laser device which attains laser oscillation at an extremely low threshold current level.
2. Description of the prior art:
Conventional semiconductor laser devices are classified into two groups, gain-guided semiconductor laser devices and index guided semiconductor laser devices, according to their optical waveguiding mechanism. Index guided semiconductor laser devices are superior to gain-guided semiconductor laser devices in view of transverse mode stabilization which is important in practical use. Index guided semiconductor laser devices having a variety of structures have been proposed, typical examples of which are BH (buried heterostructure) lasers and VSIS (V-channeled substrate inner stripe) lasers.
FIG. 7 shows a conventional BH laser device, in which a double-heterostructure with a laser-oscillating active layer 3 sandwiched between the cladding layers 2 and 4 is formed into a mesa on a substrate 1 and a burying layer 14 having a low refractive index is buried outside of the mesa. The reference numeral 5 is a cap layer 5. The BH laser device oscillates a laser beam according to an index waveguiding operation and has a low threshold current of 10 mA or less. However, if a proper refractive index is not applied to the burying layer 14 and if a proper width w is not applied to the waveguide, the device will oscillate in a high-order transverse mode. Thus, the BH laser device is disadvantageous in that it is restricted by production conditions. Moreover, in order for the BH laser device to oscillate in a fundamental transverse mode, the width of the waveguide must be set to the 2 .mu.m or less, which causes breakdown of the facets at a relatively low output power level, so that mass-production of the device cannot be attained and reliability of the device is decreased.
FIG. 8 shows a conventional VSIS laser device, which is produced as follows: On a substrate 1, a current blocking layer 6 is formed. Then, a striped V-channel having the width w is formed in the substrate 1 through the current blocking layer 6, resulting in a current path. Then, on the current blocking layer 6 including the V-channel, a cladding layer 2, a flat active layer 3 and a cladding layer 4 are successively formed, resulting in a double-heterostructure multi-layered crystal for laser oscillation operation. Even when the width w of the waveguide is set at a value of as large as 4-7 .mu.m, since a laser beam outside of the waveguide within the active layer 3 is absorbed by the substrate 1, high-order mode gain is suppressed and a high-order transverse mode does not occur. However, the threshold current of this VSIS laser device is 40-60 mA, which is extremely higher than that of the BH laser device. This is because current injected into the device is confined within the inner striped structure formed by the current blocking layer 6, but carrier injected into the active layer 3 diffuses into the outside of the active layer 3, resulting in carrier unusable for laser oscillation. FIG. 9 shows the distribution of carrier density in the junction direction y within the active layer of the VSIS laser device, indicating that when the waveguide width w is 4 .mu.m, carrier in the shaded areas (corresponding to the outside of the waveguide) is unusable for laser oscillation. The unusable carrier results in unnecessary light and/or generates unnecessary heat, causing an increase in the threshold current of the device and a decrease in reliability of the device.
In order to solve the problems of both the BH laser device and the VSIS laser device, as shown in FIG. 10, a method by which grooves are formed on both sides of the V-channel of the VSIS laser device from the cap layer 5 to the current blocking layer 6 by an etching technique, and subsequently filled with a burying layer having a greater energy gap than the active layer has been proposed by, for example, Japanese patent application No. 60-78004. The device with such a structure is hereinafter referred to as a BH-VSIS laser device, in which the diffusion of carrier in the transverse direction within the active layer is prevented by the burying layer and moreover a laser beam produced in the active layer is absorbed by the area outside of the striped channel of the substrate, resulting in a decrease in the effective refractive index, so that the portion of the active layer corresponding to the channel forms an optical waveguide, which causes the suppression of the occurrence of a high-order mode. However, in the conventional BH-VSIS laser device, leakage current I1 flows from the mesa to the burying area outside of the mesa as shown in FIG. 5, resulting in a limitation of the decrease in the threshold current. Output power at which the facets of the BH-VSIS laser device is broken down is 20-30 mW. This is the same as that of the VSIS laser device. In addition, three crystal growth processes are required for the production of the BH-VSIS laser device because of the growth of each of the current blocking layer 6, a double-heterostructure multi-layered crystal disposed on the current blocking layer and the burying layer buried outside of the mesa. A decrease in the number of crystal growth processes is desirable in view of the reproducibility and/or the mass production of the laser device.